The present invention relates to microelectronic substrates such as multi-layer circuit panels. Multi-layer circuit panels are used throughout the microelectronic arts for purposes such as mounting and interconnecting other components. For example, multiple semiconductor chips can be mounted to a single circuit panel and contacts on the various chips may be interconnected with one another by electrically conductive features of the circuit panel. Typically, the circuit panels are regarded as having “horizontal” directions, also referred to as the “x” and “y” directions parallel to the exposed component and mounting surfaces of the panels. Such circuit panels typically include several dielectric layers such as unreinforced or reinforced epoxy, polyimide or the like. The circuit panels also include electrically conductive features such as elongated electrical conductors commonly referred to as “traces” extending along surfaces of the dielectric layers, so that each layer of traces is disposed at the juncture between a pair of adjacent dielectric layers or else is disposed on an exposed surface, at the top or bottom of the stack. One widely used arrangement has many of the traces within each layer extending generally parallel to one another. Thus, the majority of traces in one layer may extend in the first or x horizontal direction, whereas the majority of traces in another layer may extend in a perpendicular or y horizontal direction. The various traces can be interconnected with one another by vertical conductors or “vias” extending in the vertical or z-direction. As described, for example, in commonly assigned U.S. Pat. Nos. 5,367,764 and 5,282,312, the disclosures of which are hereby incorporated by reference herein, vertical or z-direction connectors may be provided in various locations within such a multi-layer circuit panel so as to provide the desired pattern of connections within the traces of the various layers. Such pattern may include complex conductive paths extending along traces of various layers and extending between the layers along the vias.
Typically, the various dielectric layers, with the traces thereon and with vias extending through each individual layer are laminated together so as to form the composite, multi-layer structure. Various techniques are used to provide vertical conductors or vias extending through a plurality of dielectric layers. For example, special connecting layers may be formed from a dielectric material with adhesives or bonding agents on opposite sides of the connecting layer. The connecting layers are provided with vias extending through them. These vias may have electrically conductive bonding materials on their exposed surfaces. The connecting layers are placed into the laminate when the various layers are laminated to one another. The vias of the connecting layers can connect features on the adjacent layers to one another. Systems of this type, however, generally require care in assembly so as to assure reliable interconnections. In particular, because the adhesives on the connecting layers are activated at the same time as the conductive bonding materials used to make the electrical connections, the adhesives can be introduced into the electrical connections and can interfere with reliable connections. Although solutions to this problem are disclosed in the aforementioned '367 and '312 patents, it would be desirable to provide additional methods of interconnection which avoid the need for such solutions.
Moreover, vias of this type consume significant space within the assembly. Typically, the conductive vias in the connecting layers have flanges with diameters on the order of 0.010 inches, i.e., about 250 micrometers. The flanges engage corresponding flanges on the mating surface of the next dielectric layer in the stack. The area occupied by the flanges must remain free of traces. Moreover, the areas immediately adjacent the flanges on the mating surface must also remain free of traces to avoid accidental short-circuiting due to misalignment between the layers. The space consumed by vertical connections made by using vias and connecting layers substantially reduces the number of traces which can be provided on the surfaces of the dielectric layers. Stated another way, the ability of the panel to provide routing between the various electrical components to be connected is diminished.
In another method, the structure can be drilled after laminating the layers so as to form a vertical hole extending at least partially through the assembly. After drilling, a conductive material is deposited into the holes to form vertical connections between layers. The drilling procedure suffers from numerous drawbacks, including significant loss of space within the assembly. To form a vertical connection between two layers in the middle of the laminate, a hole must be drilled from an exposed surface of the laminate through all of the overlying layers so as to reach the layers to be connected. The layout of traces on all of the overlying layers must be arranged to provide clearance for such a hole and to accommodate the tolerances inherent in the hole drilling procedure. Moreover, the hole drilling procedure is expensive and slow. Consequently, the hole drilling procedure often is used to provide only a few connections as, for example, ground or power connections between layers.
All of the foregoing problems are becoming progressively more acute as the trend towards smaller circuit panels, with finer traces continues.